1. Field of the Invention
This invention relates mainly to a voltage regulator using a saturable transformer, and particularly to a voltage regulator having superior constant voltage characteristic.
2. Description of the Prior Art
In a resonance circuit using a parametric oscillation shown in FIG. 1, when an inductance L is varied with a frequency which is twice the resonance frequency of this circuit, there is generated an oscillating current of a frequency equal to the resonance frequency. That is, if the inductance L is periodically changed with an exciting factor m expressed as follows: EQU L=L.sub.o (1+m cos 2.omega.t)
where
m=L'/L.sub.o (exciting factor) PA1 Q=.omega.L.sub.o /r, .omega.=2.pi.f PA1 r=internal resistance of resonance circuit, PA1 K: form factor PA1 .omega.: exciting angular frequency PA1 S: effective sectional area of core wound with aforesaid winding PA1 B.sub.s : effective maximum magnetic flux density of the aforesaid core
this circuit oscillates at an angular frequency .omega. when m&gt;2/Q. The oscillating energy can be obtained as an output.
In this case, if the inductance L includes a saturated range (non-linear range) as shown in FIG. 2, the oscillating output is limited by the above non-linearity and hence a constant voltage output can be produced. An output voltage E.sub.o at this time is expressed as follows: ##EQU1## where N: number of turns of winding having inductance L
Accordingly, if a transformer having a saturated range is used to perform parametric oscillation, for example, a DC-DC converter can be formed and also a constant voltage output can be produced.
In this case, however, when a silicon steel plate, permalloy or the like is used as a core material of the transformer, an exciting frequency f must be lowered to, for example, 50 Hz to 400 Hz for reducing eddy currents. Therefore, in order to provide an output having a certain magnitude, the sectional area S of core of the transformer or the number of turns, N, of the winding must be increased as apparent from the above equation. As a result, the transformer becomes large in size and heavy in weight so that the converter also becomes large in size and heavy in weight.
On the other hand, when a ferrite is used as the core material, the exciting frequency f can be taken as high as 15 KHz to 100 KHz. Therefore, the transformer can be made small in size and weight thereby to make the converter small in size and weight, too. However, the ferrite material has a drawback that if hysteresis loss causes heat generation, the maximum magnetic flux density B.sub.s of the core is greatly changed, for example, its variation .DELTA.B.sub.s becomes about 30% for the temperature variation of 0.degree. C. to 100.degree. C. As a result, the output voltage E.sub.o will be greatly changed.
Thus, in the prior art, a ferrite material is used as the core, and the exciting frequency f is controlled or another constant voltage circuit is added to make the output voltage E.sub.o constant. By these methods, however, the control range is narrow and the construction becomes complicated.